Work on the structure and function of the contractile proteins of muscle is in progress. The mechanism of self-association of myosin molecules and the forces involved in stabilization of myosin filaments in solution is under study using various physico-chemical techniques to probe the sites of interaction and the three dimensional organization of myosin molecules within the filament. Native thick filaments of rabbit skeletal muscle have been isolated free of I-band filaments and their composition and molecular properties are currently being investigated. Other studies include an investigation of the geometry of the fundamental building unit of the thick filament, the myosin dimer, as well as the kinetics of filament assembly. A structural protein, the M-line protein, which plays an important role in the spatial organization of the thick filament lattice of muscle, has been isolated in pure form and its sub-unit structure, physico-chemical properties and interaction characteristics with myosin filaments is under examination.